1. Field of the Invention
The present invention is directed generally to the control of vibration in structures, and more particularly to the confinement of vibrational energy to selected portions of structures, removing vibrational energy from these selected portions.
2. Description of Related Art
The suppression or control of vibration has an increasing importance in the design, manufacture, operation, maintenance, precision, and safety of structures and machinery. Engineering systems are subjected to numerous disturbances from either internal or external sources of vibration. Conventional methods for reducing the effect of vibration take several forms, and may be classified into the three general categories, viz. 1) isolation, e.g. the use of rubber shock mounts, 2) absorption (redirection), and 3) suppression (dissipation). Conventional active vibration control methods utilize sensors, signal processing, actuators, and power sources to produce forces or strains in the system that counteract the vibration or effectively increase the dissipation in a system.
Many methods have been developed for adding energy dissipating (damping) mechanisms to vibrating systems. Some of the devices for adding passive damping include pneumatic and hydraulic dashpots, fluid layer dampers, viscoelastic and constrained viscoelastic layers, internal and contact mechanical friction, particle damping, impacting masses, magnetic damping, eddy current damping, and piezoelectric dissipation.
Many of the above passive methods may also be made active by enabling the control of specific material or geometric properties of the damping mechanism. Other devices for adding active damping include active constrained layer damping and closed-loop actuator-based damping methods. Closed-loop-based damping methods include feed-back and feed-forward approaches in which sensors are used to determine the vibration state of the structure, and forces dependent upon the sensor output are applied to the structure via an actuator. These forces in turn cancel or dissipate the vibration energy in the structure. "Smart" materials and structures have extended the range of active, as well as passive, vibration control mechanisms, where the term "smart" refers to materials or structures that respond to environmental or operational conditions by altering their material, geometric, or operational properties. Such a response may be triggered both with and without additional control mechanisms (such as a sensory and feed-back loop). Examples of smart materials include piezoceramics, shape memory alloys, electrostrictive and magnetostrictive materials, and rheological and magnetological fluids.
Damping methods incorporating semi-active and hybrid approaches have also been devised. In semi-active approaches, the passive or active damping mechanism may be included or excluded from the control mechanism of the structure based on the response of the structure. The determination of when the damping elements are active may be made in either an active or a passive manner.
Although active control methods have been shown to be effective in some limited applications, their drawbacks are emphasized by a reliance on computationally complex control algorithms, high numbers of sensors and high actuator power requirements, and continuous monitoring and feed-back or feed-forward mechanisms. These drawbacks have demonstrated the need for an alternative or additional approach to vibration control. Additionally, semi-active control techniques reduce only the requirement on continuous actuation but their development and implementation has not yet progressed as far as fully active control or passive control.
There are common features among the above methods. First, they are designed to dissipate vibrations in a reactive manner. The vibration control mechanism acts upon the vibration energy to suppress vibration. Second, these methods are all designed to be most effective in a certain frequency range. Isolators, absorbers, and dampers, whether active or passive, are tuned to a specific frequency range of interest. Active cancellation methods are also limited in their effective frequency range by the speed of signal processing and actuator response time requirements. Third, these methods are designed without regard to the distribution of vibrational energy throughout the system.
It is important for the economic operation and practical implementation of active and passive vibration control technologies that the number of controlled regions and controlling components be reduced so as to achieve the vibration control objectives more effectively and efficiently.
Therefore, there is a need for a method of controlling and dissipating vibrational energy in a system which is proactively designed into the system, and which takes account of total energy distribution throughout the system. There is also a need to expand the frequency range over which vibrational energy is controlled and dissipated. Further, economic considerations drive a need to reduce the number of controlled regions and controlling components and to reduce the complexity of active vibration control systems.